The glass transition temperature (Tg), crystalline melting point (Tm) and degree of crystallinity are key parameters in determining the thermo-mechanical properties of polyesters. Previous studies have succeeded in increasing the Tg of thermoplastic polymers, primarily homopolymers, but this has typically been accompanied by a corresponding increase in the Tm. Such increases in Tm can be disadvantageous because a thermoplastic polymer should also remain melt-processible (for instance in an extruder), and should preferably remain so under economic conditions (for instance, below about 320° C., preferably below about 300° C., which allows the use of conventional extrusion equipment). At higher processing temperatures, polymer extrusion requires expensive specialist equipment and a great deal of energy, and typically also results in degradation products. The melt-processing temperature should be well below (for instance, at least about 20° C. below) the decomposition temperature of the polymer. In some cases, comonomers have been introduced into polymers in order to increase Tg while retaining Tm, but also resulting in convergence of the decomposition temperature and the Tm, which leads to the production of degradation products in the melt.
Many attempts have also been made to enhance the glass transition temperature of polyesters by the introduction of more rigid comonomers. However, such comonomers also disrupt the packing of the polymer chains in the crystal lattice, so that while the Tg increases, the Tm and degree of crystallinity typically both decrease as the proportion of comonomer increases, leading ultimately to amorphous materials. In order to fabricate articles from polymeric materials, it is often critical that the polymer exhibit crystallinity to achieve articles with acceptable thermo-mechanical properties.
Poly(ethylene terephthalate) (PET) is a semi-crystalline copolymer having a glass transition temperature (Tg) of 78° C. and a crystalline melting point of (Tm) of 260° C. Poly(ethylene naphthalate) (PEN) is a semi-crystalline copolymer having a higher glass transition temperature (Tg=120° C.) relative to PET, although their crystalline melting points do not differ greatly (Tm=268° C. for PEN). The thermo-mechanical stability of PEN is significantly greater than that of PET. Many of the attempts made to enhance Tg by the introduction of more rigid comonomers have focussed on PET, which is significantly cheaper than PEN. There are no commercially available semi-crystalline polyesters with a Tg higher than PEN. Polyether ether ketone (PEEK) is one of the few examples of a high Tg (approximately 143-146° C.) semi-crystalline thermoplastic polymer, and has been used successfully in engineering and biomedical applications. However, PEEK is suitable only for certain types of articles; for instance, it is not suitable for the manufacture of biaxially oriented films. PEEK is also very expensive and has a high crystalline melting point (approximately 350° C.).